In helicopter flight control systems, a collective stick is utilized to govern the collective pitch of the main rotor blades. Typically, one end of the stick is pivotally attached to the airframe at a point to the pilot's left side, and the other end of the stick terminates in a handle within easy reach of the pilot's left hand. This allows for the pilot to grasp the handle and move the collective stick in an up or down direction which will cause the rotor blades to respond in a manner necessary to execute a flight maneuver.
A resistive force means, typically a collet assembly encircling a section of the shaft of the collective stick, is provided in order to impart a resistive force to the movement of the collective stick, and maintains the collective stick in the position last established by the pilot. Generally, this resistive force means is variable, allowing adjustment of the amount of resistive force imparted on the collective stick. In one embodiment, of the collet assembly, the variation is accomplished by means of a rotating collar which is grasped and rotated to establish a desired resistive force. The resistive force is often varied by the pilot to establish what the pilot feels is an acceptable resistance force opposing the force exerted by his hand on the collective stick.
The resistive force means satisfies two requirements. The first requirement is that the collective stick be maintained in the position last established by the pilot executing a flight maneuver. The second requirement is that there be a minimum resistance force to be overcome before the collective stick responds to any input from the pilot. This second requirement assures that pilot induced oscillations, or "PIO's", are not created by the pilot. PIO's are vertical oscillations of the aircraft resulting from flight instabilities that cause the pilot's body (notably his hand) to oscillate. When the pilot's hand is on the collective stick, the oscillations are transmitted to the control system where they are amplified in a vicious cycle.
A minimum resistive force for alleviating PIO's is determined by observing the flight characteristics of the aircraft and the characteristic response of the control system used and, once determined, is set on the resistive force means by the ground crew using an instrument. One such instrument, is a "stick force indicator", ("Stick-Force Indicator, Range 0-50 lbs" Brooklyn Tool & Machine Co. Inc., 380-384 Canal Street, New York, N.Y.) which is a springloaded force measuring device, suitable to be placed against the underside of the collective stick. A dial at one end of the stick force indicator indicates the force exerted on the collective stick via the stick force indicator, which aids the ground crew in setting the minimum resistance during normal maintenance procedures.
However, due to the ease with which the resistive force may be altered by the pilot, and an observed propensity among pilots to set the resistive force according to their own desire, it is readily appreciated that setting the resistive force means on the ground to a predetermined minimum resistance force so as to eliminate the possibility of PIO's is futile.